Sipuncula

Sipuncula is a phylum of unsegmented marine worms, commonly called peanut worms. They live in burrows or empty shells and feed with an extensible introvert bearing a mouth and tentacles.

Introvert (zoology): An extendable, retractable anterior body region found in some invertebrates. Sipunculans thrust this structure out to gather food, then withdraw it for protection.
Bilateria: The animal clade whose members develop bilateral symmetry and three germ layers. Sipunculans belong to this broad group of animals with bilaterally organized bodies.
Annelida: A phylum of segmented worms that includes earthworms, leeches, and polychaetes. Sipunculans lack visible segmentation but are nested within Annelida in modern classifications.
Infauna: Animals that live within sediment or other submerged substrates. Burrowing sipunculans are members of seafloor infaunal communities.
Sipunculus nudus: A large, widely distributed peanut worm found in marine sediments. This familiar species illustrates the typical burrowing body plan of sipunculans.
Coelom: A fluid-filled body cavity completely lined by mesoderm in many animals. Fluid in the coelom helps support the worm's body and power introvert retraction.
Marine invertebrate: An animal living in the sea that lacks a vertebral column. Sipuncula is one lineage within the diverse marine invertebrates.
Lophotrochozoa: A major animal clade that includes mollusks, annelids, and several other groups. Molecular studies place sipunculans among these protostome lineages.
Bioturbation: The mixing and disturbance of sediment by living organisms. Their burrowing and feeding redistribute sediment and organic material.
Phascolosoma arcuatum: A small peanut worm commonly found in crevices and coral rubble. Its use of narrow cavities shows how sipunculans occupy varied sheltered habitats.

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NemerteaCompared with: Peanut worms retract the whole introvert rather than deploy a separate rhynchocoel proboscis.